The present disclosure relates to a lithographic apparatus and method for preventing exposure of a peripheral portion of a substrate.
For example, U.S. Pat. No. 6,680,774 B1 describes a method and apparatus for mechanically masking a workpiece to form exposure exclusion regions. The apparatus includes a mask that is arranged atop the photosensitive surface of the workpiece. The mask is opaque to the wavelength of radiation that activates the photosensitive workpiece surface. The mask is placed onto the workpiece prior to lithographic exposure of the workpiece so that the photosensitive material (e.g., negative acting photoresist) can be removed from select regions of the workpiece to provide, for example, electrical contact directly to the workpiece upper surface. e.g. mask edge to make electrical contact.
However, placement of the mask in U.S. Pat. No. 6,680,774 requires the ring to be transferred above and across the width of the substrate. This action requires a relatively large movement of the mask relative to the substrate which provides a time constraint into the operation, especially as the mask must be placed on the substrate following loading onto a substrate support and subsequently removed before the substrate can be unloaded. Furthermore, as the mask is transferred across the substrate there is always a risk of contamination of the substrate, for example from dust or other particles falling from the mask.
To remedy these problems, U.S. Pat. No. 7,936,447 describes a mask that includes a plurality of discrete segments arranged to form a continuous ring shaped mask positioned between an outer region of a substrate and an illumination system. A mask support is connected to the substrate table and configured to couple the mask to the substrate table, the mask support comprising a mask drive mechanism arranged to adjust the position of the mask relative to the substrate table in a direction substantially perpendicular to the substrate table. However the added mass of the mask and drive mechanism may negatively impact movement of the substrate table. Alternatively, when a ring shaped mask, segmented ring, or a diaphragm arrangement is positioned proximal to the reticle, the ring needs to move in synchronicity with the substrate. This may be more difficult since the projection system typically reduces the image size of the reticle, and in that case the ring needs to be bigger and requires larger strokes of movement. The known mechanisms may thus negatively impact cycle time e.g. of a lithography stepper device.
Alternatively, U.S. Pat. No. 7,777,863 describes a substrate table comprising a substrate support surface and a rail which at least partly circumnavigates the substrate support surface. A mask to prevent exposure of a peripheral exposure region of the substrate is attached to a moveable carrier arranged to travel along the rail which circumnavigates the substrate support surface. The moveable carrier may be configured to be moveable towards and away from the center of the substrate table, such that the mask may overhang the peripheral exposure region of the substrate during exposure of the substrate by the lithographic apparatus, and may then be moved away so that it no longer overhangs the substrate, thereby allowing the substrate to be easily removed from the substrate table. However, the mask and drive mechanism may still negatively impact cycle time, since these elements add mass and footprint to the wafer table.
It is desired to provide an improved lithographic apparatus and method for preventing exposure of a peripheral portion of a substrate while maintaining substrate cleanliness with minimal impact on cycle time.